The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Engine designers and engineers are constantly challenged to innovate products in order to meet ever demanding emissions and fuel economy targets.
In a traditional valve train system, as a camshaft rotates, it presses against a flat or roller follower surface, which reciprocates to open and close intake and exhaust valves. The interface between the camshaft lobe and the mating follower experiences severe loading whether it is a sliding or rolling type. Although care is taken to well lubricate this interface, instances such as cold start provide opportunities when it is likely that oil will be scarce at times. In such cases, the cam will begin turning before pressure is sufficient to pump oil to the top of the engine. With time these components wear significantly during the life of an engine to require periodic adjustment or the use of self-adjusting hydraulic elements. A basic configuration of a typical valve train is illustrated in FIG. 1.
As fuel economy has become increasingly important, whether overhead valve (OHV) design or overhead cam (OHC) design, engine manufacturers have gone to valve train systems that use rolling contact between camshaft lobes and followers, although some still use sliding follower designs. Conventional camshaft material, such as hardened gray cast iron (GCI), works well under sliding only conditions, and it lacks the necessary material strength under high rolling contact stresses. The same is true for chilled cast iron (CCI) cams. Nodular cast iron (NCI) camshaft material has been used primarily in roller follower type valve trains. In some engine designs, the cam follower interface has also changed from sliding contact type to a roller rocker type, to reduce valve train friction. Thus use of nodular cast iron, as a higher strength substitute for CCI and GCI camshaft material, under sliding contact conditions was never fully explored until this point. No one material for camshafts has been known to meet all the requirements simultaneously under rolling and sliding contact loads, without the use of surface coating or modification on the camshaft or on the mating follower surface.
Cam phasing and variable valve actuation (VVA) are relatively new technologies that attempt to further fine tune fuel economy gains by altering the opening and closing of the valves. Use of rolling only action for both high and low lift conditions for a 2-step VVA mechanism is preferable in an OHC engine as it reduces overall valve train friction. Traditionally, camshafts subjected to predominantly sliding loads, were made from hardened GCI or CCI, and ran against alloyed CCI followers. In a 2-step camshaft design configuration, these cam materials are not expected to sustain the rolling loads, which are typically above 1400 MPa, as it exceeds the materials strength limits. If the design architecture allows use of rolling only interface for variable value actuation configurations then the materials choice is relatively simple. Steel cams under rolling loads work just fine, provided there is ample real estate for the design to work. However, they exhibit poor response due to adhesive wear under sliding conditions against traditional follower materials.
If design constraints for an overhead cam engine rule out the rolling only option for a 2-step VVA architecture, then a unique valve train design option, consisting of a lobe tri-pack subjected to rolling and sliding loads from the follower elements, is possible. The cam lobe section in contact during the high lift mode is subjected to the sliding loads, whereas during the low lift mode the other lobe section experiences the rolling load.
Assembled camshafts with tailor made lobe materials such as powder metal lobes can be used to handle loading both rolling and sliding, however, it increases the system cost. Valve spring loads, valve lifts, real estate available as well as the performance desired, dictates the use of specific valvetrain architecture employed by any specific engine manufacturer.
Due to lack of data regarding sliding wear resistance of nodular cast iron, diamond-like coating (DLC) on the follower elements subjected to sliding is a safe but expensive choice for switchable roller finger follower (SRFF) mechanisms. Thus, it is desirable to find a materials combination for camshaft lobes and follower elements that will withstand the sliding loads without having to coat the sliding elements of the followers with a diamond-like-coating or having to use powder metal lobes having tailor-made chemistries and microstructures.